High linearity doherty communication amplifier with bias control

ABSTRACT

The present invention relates to bias control of a power amplification circuit of a mobile device for improving the efficiency and the linearity properties of the power amplifier. In one embodiment, the power amplifier improves these properties by receiving a voltage control signal to bias a supplemental amplifier so that the power amplifier operates in a Doherty mode in a low output power range and in a non-Doherty mode in a high output power range. In the non-Doherty mode, the supplemental amplifier is biased as a class AB amplifier via the received voltage control signal to satisfy the non-linear operational requirements of the power amplifier in the high output power range. The power amplifier generates the voltage control signal based upon power levels of signals received from a remote base station.

RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 10/432,553 filed on May 21, 2003 entitled “PowerAmplification Apparatus of Portable Terminal,” which is National Stageapplication for and claims priority to International Application No.PCT/KR02/00163, filed Feb. 4, 2002, which claims priority to KoreanUtility Patent Application No. 2002-5924, filed on Feb. 1, 2002, all ofwhich are incorporated by reference herein for all purposes. The presentinvention is also related to U.S. patent application Ser. No. 10/737,364entitled “High Linearity Doherty Communication Amplifier with PhaseControl” filed Dec. 15, 2003, and U.S. patent application Ser. No.10/737,476 entitled “High Linearity Doherty Communication Amplifier withIntegrated Output Matching Unit” filed Dec. 15, 2003.

TECHNICAL FIELD

The present invention relates to a power amplification circuit for usein wireless communication technologies, and more particularly to a poweramplifier circuit in a mobile handset.

BACKGROUND ART

As mobile handsets used for wireless communication services are becomingsmaller and lighter, battery size and power is also decreasing.Consequently, the effective talk time (i.e., transmission time) ofmobile computing devices, mobile phones, and the like (i.e., handsets)is reduced.

In a conventional mobile handset, the Radio Frequency (RF) poweramplifier consumes most of the power consumed in contrast to the overallsystem of the mobile handset. Thus, the RF power amplifier having a lowefficiency typically results in degradation of the efficiency for theoverall system, and accordingly reduces the talk time.

For this reason, much effort has been concentrated on increasingefficiency of the RF power amplifier in the field of poweramplification. In one approach, a Doherty-type power amplifier has beenintroduced recently as a circuit for increasing efficiency of the RFpower amplifier. Unlike other conventional power amplifiers, whoseefficiency is low over the low output power range, the Doherty-typepower amplifier is designed to maintain an optimum efficiency over awide output power range (e.g., in low, intermediate, and high outputpower ranges).

A common Doherty-type power amplifier design includes both a carrier anda peak amplifier. The carrier amplifier (i.e., power or main amplifier),which is composed of relatively small transistors, operates to maintainthe optimal efficiency up to a certain low output power level. The peakamplifier (i.e., supplemental or auxiliary amplifier) operates incooperative fashion with the carrier amplifier to maintain a highefficiency until the power amplifier, as a whole, produces a maximumoutput power. When the power amplifier operates within a low poweroutput range, only the carrier amplifier is operational; the peakamplifier, being biased as a class B or C, does not operate. But, whenthe power amplifier operates within a high power output range, the peakamplifier is active and may introduce nonlinearity into the overallpower amplifier since the peak amplifier is biased as a highly nonlinearclass B or class C amplifier.

Theoretically, the above-mentioned Doherty-type power amplifier isdesigned to operate while meeting the linearity specification over anentire output power range and where high efficiency is maintained.However, as described above, because the Doherty-type power amplifiercomprises a carrier amplifier and a peak amplifier that operate witheach other, the Doherty-type power amplifier in practice does notsatisfy the linearity specification (e.g., in terms of phase or gaincharacteristics) over the entire output power range where highefficiency is maintained.

In summary, in the above-mentioned Doherty-type power amplifier in therelated art, the linearity characteristics of such a power amplificationdevice are difficult to predict, which makes it difficult to improvesuch linearity characteristics because the peak amplifier is biased at arelatively constant, low DC current level, such as a current to set thepeak amplifier as a class B or C amplifier.

BRIEF SUMMARY OF THE INVENTION

There is a need to overcome the drawbacks of the prior art and toprovide at least the advantages described hereinafter. In order to solvethe above problems pertaining to the previous technology, a specificembodiment of the present invention provides a power amplifier in amobile handset that improves efficiency and linearity by applying avoltage control signal to a peak amplifier to bias the peak amplifier.Typically, a baseband modem chipset generates the voltage control signalaccording to power levels of signals received from a base station.Specifically, in a low output power range, a control voltage in a firststate is applied to the peak amplifier so that the power amplifier isoperated in a Doherty mode and, in the high output power range, acontrol voltage in a second state is applied to the peak amplifier so asto sufficiently manage the non-linearity characteristic of the poweramplifier. In one embodiment of the invention, the voltage controlsignal in the first state is a high voltage state signal, and thevoltage control signal in the second state is a low voltage statesignal. In another embodiment of the invention, the voltage controlsignal in the first state is the low voltage state signal, and thevoltage control signal in the second state is the high voltage statesignal.

The power amplifier in a mobile handset according to one embodiment ofthe present invention comprises a phase shifter, coupled to inputterminals of a carrier amplifier and a peak amplifier, for generating aphase difference between carrier amplifier and peak amplifier inputsignals to compensate for the phase shift at an output of carrier andpeak amplifiers; and an output matching unit for transmitting the outputpowers from the carrier amplifier and the peak amplifier to an outputstage. Furthermore, the peak amplifier includes a voltage control unitconfigured to receive the voltage control signal and bias the peakamplifier in accordance with the power levels of signals received fromthe base station.

In one embodiment, the phase shifter is implemented with a 3 dB hybridcoupler, for example, for distributing certain input powers to thecarrier amplifier and the peak amplifier, minimizing interferencebetween the carrier amplifier and the peak amplifier and transmittingsignals in such a manner that the phase of input power applied to thepeak amplifier is substantially 90° delayed from the phase of inputpower applied to the carrier amplifier.

In another embodiment, the phase shifter is a phase differencecompensator connected in between the input stage of the power amplifierand the peak amplifier, for delaying the phase of input signal appliedto the peak amplifier by 90° from the phase of input signal applied tothe carrier amplifier.

The voltage control unit controls a DC bias current of the peakamplifier via the voltage control signal such that the power amplifieris operated in a Doherty mode if the power amplifier operates within thelow output power range. On the other hand, if the power amplifieroperates within the high output power range, the voltage control unitcontrols the DC bias current of the peak amplifier via the voltagecontrol signal such that the power amplifier satisfies non-linearitycharacteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the structure of a power amplifier ina mobile handset in accordance with one embodiment of the presentinvention;

FIG. 2 shows an equivalent circuit of a 3 dB hybrid coupler that can beused in the power amplifier of FIG. 1;

FIG. 3A is a block diagram of the carrier amplifier illustrated in FIG.1, according to one embodiment of the invention;

FIG. 3B is a block diagram of the input matching unit illustrated inFIG. 3A, according to one embodiment of the invention;

FIG. 3C is a block diagram of the inter-stage matching unit illustratedin FIG. 3A, according to the present invention;

FIG. 3D is a block diagram of the first stage amplifier illustrated inFIG. 3A, in accordance with one embodiment of the invention;

FIG. 3E is a block diagram of the second stage amplifier illustrated inFIG. 3A, according to one embodiment of the invention;

FIG. 4A is a block diagram of the peak amplifier illustrated in FIG. 1,according to one embodiment of the invention;

FIG. 4B is a block diagram of the second stage amplifier/voltage controlunit illustrated in FIG. 4A, according to one embodiment of theinvention;

FIG. 4C is a block diagram of the second stage amplifier/voltage controlunit illustrated in FIG. 4A, according to another embodiment of theinvention;

FIG. 4D is a block diagram of the second stage amplifier/voltage controlunit illustrated in FIG. 4A, according to yet another embodiment of theinvention;

FIG. 5 is a block diagram of the exemplary output matching unitillustrated in FIG. 1;

FIG. 6 shows an equivalent circuit of the exemplary output matching unitof FIG. 5, implemented with lumped elements;

FIG. 7 is a graph illustrating efficiency characteristics dependent on avoltage control signal applied to an exemplary peak amplifier;

FIG. 8 is a graph illustrating non-linearity characteristics dependenton a voltage control signal applied to an exemplary peak amplifier;

FIG. 9 is a graph illustrating efficiency characteristics correspondingto modes of the power amplifier in accordance with one embodiment of thepresent invention;

FIG. 10 is a graph illustrating non-linearity characteristicscorresponding to modes of the power amplifier in accordance with aspecific embodiment of the present invention;

FIG. 11 is a graph illustrating gain characteristics corresponding tomodes of the power amplifier in accordance with the present invention;and

FIG. 12 is a block diagram showing the structure of a power amplifier inaccordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Hereinafter, a detailed description will be given with reference to theattached drawings as to an exemplary power amplifier in a mobile handsetin accordance with various embodiments of the present invention.

FIG. 1 illustrates the structure of an exemplary power amplifier in amobile handset in accordance with a specific embodiment of the presentinvention. The power amplifier 100 illustrated in FIG. 1 comprises ahybrid coupler, such as exemplary 3 dB hybrid coupler 110, a carrieramplifier 120, a peak amplifier 130, and an output matching unit 140. 3dB hybrid coupler 110 distributes certain input powers to carrieramplifier 120 and peak amplifier 130, minimizes interference betweencarrier amplifier 120 and peak amplifier 130 and transmits signals insuch a manner that the phase of input power of peak amplifier 130 is 90°(λ/4) delayed from the phase of input power of carrier amplifier 120.Accordingly, the 3 dB hybrid coupler 110 compensates for a laterprocessing of output signals from carrier amplifier 120 and peakamplifier 130 by output matching unit 140 by generating a 90° (λ/4)phase delay at the output matching unit 140 between the phases of outputsignals from carrier amplifier 120 and peak amplifier 130. Thus, 3 dBhybrid coupler 110's introduction of phase difference between the phasesof output powers from carrier amplifier 120 and peak amplifier 130 tocompensate for subsequent processing of the output powers by the outputmatching unit 140 results in an equalization of the phases of the outputpowers and an optimum output power signal at an output stage 70. 3 dBhybrid coupler 110 is discussed further below in conjunction with FIG.2.

Carrier amplifier 120 amplifies signals received from 3 dB hybridcoupler 110. In one example, carrier amplifier 120 includes a transistorthat can be sized smaller than that of a transistor constituting peakamplifier 130. The ratio of these respective transistor sizes, in part,determines an output power range over which the maximum efficiency canbe maintained. The higher this ratio, the wider the output power rangeover which the maximum efficiency can be maintained. One having ordinaryskill in the art should appreciate that each amplifier can include oneor more transistors or other like circuit elements. Further, that theordinarily skilled artisan should recognize that carrier amplifier 120and peak amplifier 130 can be implemented in any known semiconductortechnologies, such as Si LDMOS, GaAS MESFET, GaAs pHEMT, GaAs HBT, orthe like. Carrier amplifier 120 is discussed further below inconjunction with FIGS. 3A–3E.

Peak amplifier 130, which is another amplifier for amplifying signalsreceived from 3 dB hybrid coupler 110, is not substantially operatedwhile low-level input signals are applied to carrier amplifier 120. Thisis made possible by applying a voltage control signal Vc to peakamplifier 130 such that peak amplifier 130 is biased as a class B or Camplifier, where little or no DC current flows. Over the low outputpower range where peak amplifier 130 is not substantially operated,carrier amplifier 120 has an output impedance having a relativelyconstant and high value. Since peak amplifier 130 does not draw anycurrent, power amplifier 100 can obtain improved efficiency at an outputpower level which is lower than the highest output power level thatcarrier amplifier 120 can generate.

Peak amplifier 130 is configured to receive the voltage control signalVc from a baseband modem chipset (not shown) or from power amplifier RFprocessing circuitry (not shown). The baseband modem chipset generatesthe voltage control signal Vc based upon power levels of signalsreceived from a base station (not shown). The power amplifier RFprocessing circuitry processes signals from the baseband modem chipset,and is well know to one skilled in the art. Peak amplifier 130 isdiscussed further below in conjunction with FIGS. 4A–4D.

Output matching unit 140 includes a first λ/4 transformer 143. First λ/4transformer 143 operates as an impedance inverter and is used to providean impedance at a carrier amplifier output terminal 50 that is invertedfrom an impedance at a peak amplifier output terminal 60. A second λ/4transformer 145 at the peak amplifier output terminal 60 of the peakamplifier 130 matches an output impedance of the power amplifier 100 toa reference characteristic impedance which is typically 50 ohms. Outputmatching unit 140 is discussed further below in conjunction with FIGS.5–6.

FIG. 2 shows an equivalent circuit of 3 dB hybrid coupler 110 inaccordance with one embodiment of the present invention. The FIG. 2embodiment of 3 dB hybrid coupler 110 comprises a plurality of lumpedelements, including a capacitor 111, an inductor 112, a capacitor 113,an inductor 114, an inductor 115, a capacitor 116, an inductor 117, anda capacitor 118. At an operating frequency of approximately 1.8 GHz, forexample, nominal capacitances of capacitors 111, 113, 116, and 118 are afew pico-Farads (pF), and nominal inductances of inductors 112, 114,115, and 117 are a few nano-Henries (nH). After signals are received byinput stage 10 of 3 dB hybrid coupler 110, which has the signal couplingof about 3 dB or more, such signals are transmitted to carrier amplifierinput terminal 30 (FIG. 1) and to peak amplifier input terminal 40 (FIG.1). The signal at carrier amplifier input terminal 30 and the signal atpeak amplifier input terminal 40 have a phase difference at or about 90°(λ/4, or quarter-wave).

As an example, 3 dB hybrid coupler 110 can be implemented with atransmission line, such as a coupled line coupler, a Lange coupler, abranch line coupler or other like coupling circuits known in the art. Asanother example, 3 dB hybrid coupler 110 may be implemented using aMicrowave Monolithic Integrated Circuit (MMIC) chip technology, such asGaAS or any other known semiconductor technologies. That is, exemplaryhybrid coupler 110 can be fabricated as an integrated circuit, which canbe packaged as a single power amplifier device or chip. In still yetanother example, 3 dB hybrid coupler 110 may be implemented by the LowTemperature Co-fired Ceramic (LTCC) method or other similartechnologies.

FIG. 3A is a block diagram of carrier amplifier 120 illustrated in FIG.1, according to one embodiment of the invention. In the FIG. 3Aembodiment of the invention, carrier amplifier 120 is a two-stageamplifier and includes an input matching unit 305, a first stageamplifier 310, an inter-stage matching unit 315 and a second stageamplifier 320. The input matching unit 305 matches an output impedanceof 3 dB hybrid coupler 110 with an input impedance of carrier amplifier120. Similarly, the inter-stage matching unit 315 matches an outputimpedance of first stage amplifier 310 with an input impedance of secondstage amplifier 320. Input matching unit 305 and inter-stage matchingunit 315 are discussed further below in conjunction with FIGS. 3B and3C, respectively.

In addition, carrier amplifier 120 includes conductor lines 325electrically coupled to a DC bias voltage V1 (not shown) and conductorlines 330 electrically coupled to a DC bias voltage V2 (not shown) forbiasing first stage amplifier 310 and second stage amplifier 320. In anexemplary embodiment of the invention, 2.8V<V1<3.0V and 3.2V<V2<4.2V,although the scope of the invention covers other bias voltages inaccordance with operating characteristics of first stage amplifier 310and second stage amplifier 320.

FIG. 3B is a block diagram of input matching unit 305 illustrated inFIG. 3A, according to one embodiment of the invention. Input matchingunit 305 includes an inductor 306, a capacitor 307 and a capacitor 308.Inductor 306 electrically couples 3 dB hybrid coupler 110 (FIG. 1) withcapacitor 307 and capacitor 308. Additionally, capacitor 307 iselectrically coupled to ground, and capacitor 308 is electricallycoupled to first stage amplifier 310 (FIG. 3A). In one embodiment of theinvention, electrical characteristics of inductor 306, capacitor 307,and capacitor 308 are selected such that an output impedance of 3 dBhybrid coupler 110 is matched to an input impedance of carrier amplifier120 (FIG. 3A), measured at a terminal 30. For example, capacitances ofcapacitors 307 and 308 are nominally a few pico-Farads, and inductor 306has a nominal inductance of a few nano-Henries.

FIG. 3C is a block diagram of inter-stage matching unit 315 illustratedin FIG. 3A, according to the present invention. Inter-stage matchingunit 315 includes a capacitor 309, an inductor 311 and a capacitor 312.Capacitor 309 electrically couples a signal received from first stageamplifier 310 (FIG. 3A) with inductor 311 and capacitor 312.Furthermore, inductor 311 is electrically coupled to ground, andcapacitor 312 is electrically coupled to second stage amplifier 320(FIG. 3A). In one embodiment of the invention, electricalcharacteristics of capacitor 309, inductor 311, and capacitor 312 areselected such that an output impedance of first stage amplifier 310(FIG. 3A) is matched to an input impedance of second stage amplifier 320(FIG. 3A). For example, capacitances of capacitors 309 and 312 arenominally a few pico-Farads, and inductor 311 has a nominal inductanceof a few nano-Henries.

FIG. 3D is a block diagram of first stage amplifier 310 illustrated inFIG. 3A, in accordance with one embodiment of the invention. First stageamplifier 310 includes a conventional bias unit 1 (CBU1) 335, aconventional bias unit 2 (CBU2) 340 and a transistor Q11 345. In theFIG. 3D exemplary embodiment of the invention, transistor Q1 345 isconfigured as a common-emitter npn bipolar transistor. CBU1 335 includesa resistor 313, a diode 314, a diode 316, a resistor 317, a capacitor318, and a transistor Q1A 319. CBU2 340 includes a transmission line 321and a capacitor 322. As known to one in the art, electricalcharacteristics of resistor 313, diode 314, diode 316, resistor 317,capacitor 318, and transistor Q1A 319, collectively referred to as firststage base bias elements for descriptive purposes, are selected inconjunction with DC bias voltages V1 and V2 to bias a base of transistorQ11 345 for normal mode of operation. For example, resistor 313 may havea resistance in a range of several hundred Ohms to several kilo-Ohms,resistor 317 may have a resistance in a range of several Ohms to severalhundred Ohms, and a Q1A:Q11 transistor size ratio may be approximatelyin a range of 1:4 to 1:10. Similarly, electrical characteristics oftransmission line 321 and capacitor 322, collectively referred to asfirst stage collector bias elements for descriptive purposes, areselected in conjunction with bias voltage V2 to bias a collector oftransistor Q11 345 for normal mode of operation. For example, electricalcharacteristics of the first stage base bias elements are selected tospecify a base-emitter current I_(BE) (not shown) of transistor Q11 345and electrical characteristics of the first stage collector biaselements are selected to specify a collector-emitter voltage V_(CE) (notshown) of transistor Q11 345, thus allowing transistor Q11 345 tooperate within a normal mode of operation and with a predefinedamplification factor.

FIG. 3E is a block diagram of second stage amplifier 320 illustrated inFIG. 3A, according to one embodiment of the invention. Second stageamplifier 320 includes a conventional bias unit 3 (CBU3) 350 and atransistor Q12 355. CBU3 350 includes a resistor 323, a diode 324, adiode 326, a resistor 327, a capacitor 328, and a transistor Q1B 329,collectively referred to as second stage base bias elements. In the FIG.3E embodiment of the invention, coupling of the second stage base biaselements of CBU3 350 is identical to coupling of the first stage basebias elements of CBU1 335 (FIG. 3D). However, electrical characteristicsof the second stage base bias elements may or may not be identical toelectrical characteristics of the first stage base bias elements. Forexample, resistor 313 (FIG. 3D) and resistor 323 may have differentresistance values, and transistor Q1A 319 (FIG. 3D) and transistor Q1B329 may be of different sizes. In operation, electrical characteristicsof resistor 323, diode 324, diode 326, resistor 327, capacitor 328, andtransistor Q1B 329 are selected in conjunction with DC bias voltages V1and V2 to bias a base of transistor Q12 355 for normal-mode operation,based upon operating characteristics of transistor Q12 355 andspecifications of power amplifier 100 (FIG. 1). For example, resistor323 may have a resistance in a range of several hundred Ohms to severalkilo-Ohms, resistor 327 may have a resistance in a range of several Ohmsto several hundred Ohms, a Q1B:Q12 transistor size ratio may beapproximately in a range of 1:4 to 1:10, and a Q11:Q12 transistor sizeratio may be approximately in a range of 1:4 to 1:8. However, the scopeof the present invention covers other transistor size ratios that arewithin operating specifications of carrier amplifier 120 (FIG. 1) andpower amplifier 100 (FIG. 1). In the FIG. 3E exemplary embodiment of theinvention, transistor Q12 355 is configured as a common-emitter npnbipolar transistor.

FIG. 4A is a block diagram of peak amplifier 130 illustrated in FIG. 1,according to one embodiment of the invention. In the FIG. 4A embodimentof the invention, peak amplifier 130 is a two-stage amplifier andincludes an input matching unit 405, a first stage amplifier 410, aninter-stage matching unit 415 and a second stage amplifier/voltagecontrol unit 420. Various embodiments of second stage amplifier/voltagecontrol unit 420 are discussed below in conjunction with FIGS. 4B–4D.

In one embodiment of the invention, input matching unit 405 isconfigured as input matching unit 305 (FIG. 3B) with electricalcharacteristics of inductor 306 (FIG. 3B), capacitor 307 (FIG. 3B), andcapacitor 308 (FIG. 3B) selected such that an output impedance of 3 dBhybrid coupler 110 (FIG. 1) is matched to an input impedance of peakamplifier 130, measured at a terminal 40. Similarly, inter-stagematching unit 415 is configured as inter-stage matching unit 315 (FIG.3C) with electrical characteristics of capacitor 309 (FIG. 3C), inductor311 (FIG. 3C), and capacitor 312 (FIG. 3C) selected such that an outputimpedance of first stage amplifier 410 is matched to an input impedanceof second stage amplifier/voltage control unit 420. Finally, first stageamplifier 410 is configured as first stage amplifier 310 (FIG. 3D) withelectrical characteristics of first stage base bias elements (i.e.,resistor 313, diode 314, diode 316, resistor 317, capacitor 318, andtransistor QIA 319), first stage collector bias elements (i.e.,transmission line 321 and capacitor 322), and transistor Q11 345 (FIG.3D) selected such that first stage amplifier 410 operates according topredefined specifications, such as gain, normal mode, and cutoff modespecifications.

FIG. 4B is a block diagram of second stage amplifier/voltage controlunit 420 illustrated in FIG. 4A, according to one embodiment of theinvention. The second stage amplifier/voltage control unit 420 includesa second stage amplifier 445 and a voltage control unit 435. Secondstage amplifier 445 is configured as second stage amplifier 320 (FIG.3E). For example, second stage amplifier 445 includes a CBU3 440 and atransistor Q22 450. CBU3 440 includes a resistor 423, a diode 424, adiode 426, a resistor 427, a capacitor 428, and a transistor Q2B 429,collectively referred to as second stage peak amplifier base biaselements. In operation, electrical characteristics of the second stagepeak amplifier base bias elements are selected in conjunction with DCbias voltages V3 and V4 to bias a base of transistor Q22 450 fornormal-mode operation, based upon operating characteristics oftransistor Q22 450 and specifications of power amplifier 100 (FIG. 1).For example, resistor 423 may have a resistance in a range of severalhundred Ohms to several kilo-Ohms, resistor 427 may have a resistance ina range of several Ohms to several hundred Ohms, a Q2B:Q22 transistorsize ratio may be approximately in a range of 1:4 to 1:10, DC biasvoltage V3 may be in a range of 2.8V to 3.0V, and DC bias voltage V4 maybe in a range of 3.2V to 4.2V. Second stage amplifier 445 receives asignal from inter-stage matching unit 415, amplifies the received signalbased upon the voltage control signal Vc received by voltage controlunit 435, and sends the amplified signal to peak-amplifier outputterminal 60.

Voltage control unit 435 receives the voltage control signal Vc(typically in a range of 2.8V to 4.2V), and controls a DC bias currentof second stage amplifier 445. In the FIG. 4B embodiment of theinvention, voltage control unit 435 includes a resistor 431 and atransistor Qc 432. Typically, resistor 431 has a resistance in a rangeof several hundred Ohms to several kilo-Ohms, and a Qc:Q2B transistorsize ratio may be approximately in a range of 1:1 to 1:8. In operation,a base station (not shown) for receiving, transmitting and processing RFsignals sends signals to a baseband modem chipset (not shown) inresponse to RF signals received from power amplifier 100. The basebandmodem chipset processes the signals, and generates the voltage controlsignal Vc. Voltage control unit 435 then receives the voltage controlsignal Vc from the baseband modem chipset. In another embodiment of theinvention, power amplifier 100 includes RF processing circuitry (notshown) for processing the signals received by the baseband modemchipset. In this embodiment, the RF processing circuitry generates thevoltage control signal Vc, and sends the voltage control signal to thevoltage control unit 435. The RF processing circuitry and the basebandmodem chipset are well know in the art, and will not be described infurther detail.

Typically, the baseband modem chipset generates the voltage controlsignal Vc based upon power levels of signals transmitted by the basestation and received by the baseband modem chipset. For example, if thebaseband modem chipset, upon receiving the signals from the basestation, determines that power amplifier 100 operates in a low poweroutput range, the baseband modem chipset sends a “high” voltage controlsignal Vc (i.e., a high voltage state signal) to voltage control unit435. However, if the baseband modem chipset, upon receiving the signalsfrom the base station, determines that power amplifier 100 operates in ahigh power output range, the baseband modem chipset sends a “low”voltage control signal Vc (i.e., low voltage state signal) to voltagecontrol unit 435. The scope of the present invention covers a voltagecontrol signal Vc corresponding to any voltage state and to any poweroutput range.

In operation, if the baseband modem chipset transmits a low voltagestate control signal Vc to peak amplifier 130 that indicates poweramplifier 100 operates in the high power output range, the voltagecontrol unit 435 receives the low voltage state control signal Vc andsets a DC bias current of second stage amplifier 445 of peak amplifier130 (FIG. 4A) via the received low voltage state control signal Vc. Thelow voltage state control signal Vc turns off transistor Qc 432,increases base-emitter currents (not shown) of transistors Q2B 429 andQ22 450, and biases peak amplifier 130 as a class AB amplifier.

However, if the baseband modem chipset transmits a high voltage statecontrol signal Vc to peak amplifier 130 that indicates power amplifier100 operates in the low power output range, the voltage control unit 435receives the high voltage state control signal Vc and sets a DC biascurrent of second stage amplifier 445 of peak amplifier 130 via thereceived high voltage state control signal Vc. The high voltage statecontrol signal Vc turns on transistor Qc 432, and diverts base-emittercurrent of transistor Q2B 429 to collector-emitter current of transistorQc 432. Thus, base-emitter currents of transistor Q2B 429 and Q22 450decrease, and peak amplifier 130 is biased as either a class B or classC amplifier, dependent upon a resultant bias state of transistor Q22450.

FIG. 4C is a block diagram of second stage amplifier/voltage controlunit 420 illustrated in FIG. 4A, according to another embodiment of theinvention. Second stage amplifier/voltage control unit 420 includessecond stage amplifier 445 and a voltage control unit 455. Second stageamplifier 445 is identically configured as second stage amplifier 445illustrated in FIG. 4B. Voltage control unit 455 includes a resistor456, a resistor 457, a transistor Qc1 458, and a transistor Qc2 459. Inaddition, a DC bias voltage V3 is applied to voltage control unit 455via a line 461. Typically, resistor 456 has a resistance in a range ofseveral hundred Ohms to several kilo-Ohms, resistor 457 has a resistancein a range of several Ohms to several hundred Ohms, a Qc2:Qc1 transistorsize ratio may be approximately in a range of 1:1 to 1:10, a Qc1:Q2B(FIG. 4B) transistor size ratio may be approximately in a range of 1:1to 1:8, DC bias voltage V3 may be in a range of 2.8V to 3.0V, a DC biasvoltage V4 may be in a range of 3.2V to 4.2V, and a voltage controlsignal Vc may be in a range of 2.8V to 4.2V.

Input/output characteristics of voltage control unit 455 are opposite toinput/output characteristics of voltage control unit 435 (FIG. 4B). Thatis, a low voltage state control signal Vc received at a terminal 61biases peak amplifier 130 (FIG. 4A) as either a class B or a class Camplifier dependent upon a resultant bias state of transistor Q22 450(FIG. 4B), and a high voltage state control signal Vc biases peakamplifier 130 as a class AB amplifier.

FIG. 4D is a block diagram of second stage amplifier/voltage controlunit 420 illustrated in FIG. 4A, according to yet another embodiment ofthe invention. Second stage amplifier/voltage control unit 420 includessecond stage amplifier 445 and a voltage control unit 460. Second stageamplifier 445 is identically configured as second stage amplifier 445illustrated in FIG. 4B. Voltage control unit 460 includes a resistor462, a transistor Qc3 463, and a transistor Qc4 464. In addition, a DCbias voltage V4 is applied to voltage control unit 460 via a line 466.Typically, resistor 462 has a resistance in a range of several hundredOhms to several kilo-Ohms, a Qc3:Qc4 transistor size ratio may beapproximately in a range of 1:1 to 1:10, a Qc4:Q2B (FIG. 4B) transistorsize ratio may be approximately in a range of 1:1 to 1:8, a DC biasvoltage V3 may be in a range of 2.8V to 3.0V, DC bias voltage V4 may bein a range of 3.2V to 4.2V, and a voltage control signal Vc may be in arange of 2.8V to 4.2V.

Input/output characteristics of voltage control unit 460 are similar toinput/output characteristics of voltage control unit 435 (FIG. 4B). Thatis, a low voltage state control signal Vc biases peak amplifier 130 as aclass AB amplifier, and a high voltage state control signal Vc biasespeak amplifier 130 as either a class B or a class C amplifier, dependentupon a resultant bias state of transistor Q22 450 (FIG. 4B).

FIG. 5 is a block diagram of output matching unit 140 illustrated inFIG. 1. By adjusting α and β (either individually or both) of first λ/4transformer 143 and second λ/4 transformer 145, respectively, in outputmatching unit 140, the characteristic impedances of the two λ/4transformer lines change. By optimizing α and β, the carrier amplifier120 may achieve the maximum efficiency at an output power level that islower than the highest output power level that carrier amplifier 120 maygenerate.

First λ/4 transformer 143 and second λ/4 transformer 145 may beimplemented with λ/4 transmission lines (T-lines), as shown in FIG. 5,or with lumped elements 143 a, 143 b, 143 c, 143 d, . . . , 145 a, 145b, 145 c, 145 d, etc., as shown in FIG. 6, or with like elements. Outputmatching unit 140 may be implemented with many different combinations ofcapacitors and inductors (143 a, 143 b, 143 c, 143 d, . . . , 145 a, 145b, 145 c, 145 d, etc.) to match a specific output impedance at outputstage 70 and generate a specific impedance at carrier amplifier outputterminal 50 that is inverted from an impedance at a peak amplifieroutput terminal 60. Alternatively, first λ/4 transformer 143 and secondλ/4 transformer 145 may be implemented by either the LTCC method or amulti-layer method. As another example, first λ/4 transformer 143 andsecond λ/4 transformer 145 can be formed as a single integrated circuit.

FIG. 7 is a graph illustrating efficiency characteristics as determinedby, for example, the voltage control signal Vc applied to peak amplifier130 (FIG. 1). Mode 0 represents the region of amplifier operation in alow output power range (i.e., from a minimum output power in dBm topoint Q). Mode 1 represents the region of amplifier operation in a highoutput power range (i.e., from point Q to point S and/or T). As acurrent is increasingly applied to peak amplifier 130, an exemplarypower amplifier according to an embodiment operates first as shown ascurve D. Curves C and B represent the efficiency characteristicsassociated with the exemplary power amplifier as the amount of DC biascurrent increases beyond that associated with curve D. Curve Arepresents the efficiency characteristics of a general power amplifier.

As current starts to flow in peak amplifier 130, peak amplifier 130commences its operation. This changes the output impedance of carrieramplifier 120, thereby optimizing efficiency of power amplifier 100 to acertain constant level as indicated by D in FIG. 7. Accordingly, asindicated by curve D in FIG. 7, the Power Added Efficiency (PAE) has themaximum value from the point P (when peak amplifier 130 starts tooperate) to either point S, which is the highest allowable output powersatisfying the given linearity conditions, or point T, which is thesaturated output power, as generated by power amplifier 100. Thus, asillustrated, improved efficiency characteristics are achieved through anexemplary power amplifier, according to an embodiment of the presentinvention, in comparison with the efficiency characteristic of a generalpower amplifier indicated by curve A in FIG. 7. As described above, thisis made possible by operating peak amplifier 130 at class B or C.

However, illustrated by the graph of FIG. 8 are non-linearitycharacteristics as the voltage control signal Vc is applied to peakamplifier 130. In this graph, performance of power amplifier 100 ischaracterized with respect to the Adjacent Channel Power Ratio (ACPR) asthe output power is increased. In this instance, values of the overallnon-linearity characteristics (as indicated by curve D in FIG. 8) may bedifficult to predict and, thus, the non-linear distortion of poweramplifier 100 becomes undesirable. Accordingly, ACPR criterion R, whichmay be required by a specific system, may not be maintained up to thedesired output power level associated with point S without violating theACPR criteria. ACPR criteria are well known and those having ordinaryskill in the art understand that R could, for example, represent −42 dBcfor a CDMA cellular system or any other value.

In other words, as illustrated in FIG. 7 and FIG. 8, compared withgeneral power amplifiers known in the related art, and if peak amplifier130 in the power amplifier 100 is operated at class B or C (that is, ifthe power amplifier 100 is operated in a typical Doherty mode), thenpower amplifier 100 shows improved efficiency characteristics overconventional power amplifiers used, for example, in wirelesscommunication applications. However, in terms of linearity, the poweramplifier might have less predictable values when operating in the highoutput power range.

Therefore, an exemplary power amplifier in accordance with an embodimentof the present invention meets high efficiency and linearityrequirements in the low output power range, such as at point Q, wherethe ACPR criterion R required by the system is satisfied. For low-powermode 0 operation, criterion R is met even if one sets the voltagecontrol signal Vc applied to peak amplifier 130 in such a way that peakamplifier 130 is operated at class B or C where little DC current flows,and thus that power amplifier 100 is operated in the Doherty mode. Onthe other hand, in the high output power range during mode 1, poweramplifier 100 can achieve excellent linearity by adjusting the voltagecontrol signal Vc applied to peak amplifier 130. This linearity can berealized by increasing the DC bias current to second stage amplifier 445of peak amplifier 130 through decrease of the voltage control signal Vcto a point where the linearity specification (or level of linearity)designated as R in FIG. 8 can be satisfied. In this way, peak amplifier130 can be biased as a class AB amplifier depending on, for example, themode of operation. This results in the efficiency and linearity curvesof B or C in FIGS. 7 and 8.

FIG. 9 is a graph illustrating efficiency characteristics correspondingto modes of power amplifier 100 (FIG. 1) in accordance with anembodiment of the present invention. FIG. 10 is a graph illustratingnon-linearity characteristics corresponding to modes of power amplifier100 in accordance with the present invention. In operation of exemplarypower amplifier 100, consider FIG. 10. When the power amplifier 100requires an output power level reaching point Q, where mode switching isneeded, the baseband modem chipset (not shown) sends a low voltage statecontrol signal Vc to peak amplifier 130 so that an increased biascurrent may be applied to peak amplifier 130. In this way, linearity ofpower amplifier 100 in accordance with an embodiment of the presentinvention is enhanced with a slight reduction in the efficiency. In oneembodiment of the invention, point Q is in a range of 15–19 dBm,however, the present invention covers other operating output powers atwhich power amplifier 100 switches modes. The efficiency and linearitycurves in mode 1 is similar to those of curves B (FIGS. 7–8). Thisprevents criteria R from being violated.

FIG. 11 is a graph illustrating gain characteristics corresponding tomodes of power amplifier 100 (FIG. 1) in accordance with the presentinvention. In the present invention, carrier amplifier 120 and peakamplifier 130 may be operated to have the same linear gaincharacteristics. However, the overall system is not affected even ifcarrier amplifier 120 and peak amplifier 130 are implemented to beoperated with different linear gain characteristics since two modes canbe distinguished clearly and be operated independently in accordancewith a specific embodiment of the present invention.

FIG. 12 is a block diagram showing the structure of a power amplifier ina mobile handset in accordance with another embodiment of the presentinvention. The power amplifier according to another embodiment of thepresent invention is substantially the same as the power amplifier 100shown in FIG. 1, in terms of the structure and operation. Therefore, thesame reference numerals refer to the same parts in the power amplifiersaccording to FIG. 1 and FIG. 12. Thus, a detailed description of thepower amplifier according in FIG. 12 is not necessary for one havingordinary skill in the art and thus is omitted.

As shown in FIG. 12, another exemplary power amplifier in accordancewith another embodiment comprises a phase difference compensator 180which replaces 3 dB hybrid coupler 110 of FIG. 1. Phase differencecompensator 180 is coupled to input stage 10 and peak amplifier 130 sothat the input signal is applied to peak amplifier 130 and to carrieramplifier 120, where phase difference compensator 180 has a phasedifference of 90° (λ/4).

As described above, because input signal applied to peak amplifier 130and input signal applied to carrier amplifier 120 has a phase differenceof 90° (λ/4) through the operation of the phase difference compensator180, when the output powers from the carrier amplifier 120 and the peakamplifier 130 join in the output matching unit 140, there would be nophase difference and thus the optimum output power may be obtained.

If phase difference compensator 180 is used instead of 3 dB hybridcoupler 110, the phase difference compensator 180 may be implementedwith one simple transmission line. Alternatively, the phase differencecompensator 180 may be implemented with lumped elements because thesimple transmission line may be approximated to inductance values. Inthis manner, the power amplifier may be implemented without a complex 3dB hybrid coupler 110 or a large-size transmission line outside of theamplifier. Furthermore, because the phase difference compensator 180 maybe integrated within a single chip and/or a single integrated circuit,the overall size of power amplifier 100 may be reduced and the price ofpower amplifier 100 may also be reduced.

In summary, when a low output power range (mode 0) generated by poweramplifier 100 of the mobile handset is adequate for proper functioningof a mobile handset/base station pair, as determined by power levels ofsignals received by the baseband modem chipset, then the baseband modemchipset sends a voltage control signal Vc in a first state to peakamplifier 130 such that power amplifier 100 is operated in the Dohertymode (i.e., so that peak amplifier 130 is operated as a class B or Camplifier). In contrast, if a low output power range (mode 0) generatedby power amplifier 100 of the mobile handset is inadequate for properfunctioning of a mobile handset/base station pair as determined by thepower levels of signals received by the baseband modem chipset, and thebase station requires power amplifier 100 to operate in the high outputpower range (mode 1), then the baseband modem chipset sends a voltagecontrol signal Vc in a second state to peak amplifier 130 such that DCbias current applied to peak amplifier 130 is increased and the ACPR isimproved up to point R where the non-linearity specification of poweramplifier 100 is satisfied. In one embodiment of the invention, thevoltage control signal Vc in the first state is a high voltage statesignal, and the voltage control signal Vc in the second state is a lowvoltage state signal. In another embodiment of the invention, thevoltage control signal Vc in the first state is the low voltage statesignal, and the voltage control signal Vc in the second state is thehigh voltage state signal.

Although several embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

As described above, an exemplary power amplifier of the presentinvention in a mobile handset that provides improves efficiency andlinearity, by controlling a DC bias current applied to a peak amplifierof the mobile handset via a control signal Vc received from a basebandmodem chipset according to relevant power levels of signals received bythe baseband modem chipset has been shown. For example, in the lowoutput power range, a state of a control signal Vc applied to a peakamplifier is selected so that the power amplifier of the presentinvention is operated in the Doherty mode and, in the high output powerrange, the state of the control signal Vc applied to the peak amplifieris selected so as to satisfy the non-linearity specification of thepower amplifier.

Various features and aspects of the above-described invention may beused individually or jointly. Further, the invention can be utilized inany number of environments and applications beyond those describedherein without departing from the broader spirit and scope of thespecification. The specification and drawings are, accordingly, to beregarded as illustrative rather than restrictive. The scope of theinvention is not limited to the described embodiments and is to bedetermined solely by the appended claims.

1. A system for bias control of a power amplifier, comprising: a carrieramplifier coupled to an input stage for amplifying an input signal; apeak amplifier coupled to the input stage for amplifying the inputsignal, the peak amplifier configured to receive a voltage controlsignal for biasing the peak amplifier, the voltage control signal basedon power levels of signals transmitted by a remote base station, and anoutput matching unit configured to receive an output signal from thepeak amplifier and an output signal from the carrier amplifier togenerate a substantially optimum power amplifier output power signal atan output stage, the output matching unit including a first quarterwavelength transformer coupled to a carrier amplifier output terminal;and a second quarter wavelength transformer coupled to a peak amplifieroutput terminal, an output of the first quarter wavelength transformer,and the output stage.
 2. The system of claim 1, wherein the carrieramplifier further comprises a carrier first stage amplifier coupled tothe input stage; and a carrier second stage amplifier coupled to thecarrier first stage amplifier and a carrier amplifier output terminal.3. A system, for bias control of a power amplifier, comprising: acarrier amplifier coupled to an input stage for amplifying an inputsignal; and a peak amplifier coupled to the input stage for amplifyingthe input signal, the peak amplifier configured to receive a voltagecontrol signal for biasing the peak amplifier, the peak amplifierincluding a peak first stage amplifier coupled to the input stage; apeak second stage amplifier coupled to the peak first stage amplifierand a peak amplifier output terminal; and a voltage control unit coupledto the peak second stage amplifier, the voltage control unit configuredto bias the peak amplifier via the received voltage control signal. 4.The system of claim 3, wherein the voltage control unit biases the peakamplifier as a class B or a class C amplifier based upon a state of thereceived voltage control signal.
 5. The system of claim 3, wherein thevoltage control unit biases the peak amplifier as a class AB amplifierbased upon a state of the received voltage control signal.
 6. The systemof claim 1, wherein the power amplifier is configured to generate thevoltage control signal in a first state if the power levels of thesignals transmitted by the remote base station indicate that the poweramplifier operates in a low output power range.
 7. The system of claim1, wherein the power amplifier is configured to generate the voltagecontrol signal in a second state if the power levels of the signalstransmitted by the remote base station indicate that the power amplifieroperates in a high output power range.
 8. The system of claim 1, furthercomprising a 3dB hybrid coupler configured to receive the input signalfrom the input stage, send a first input signal to an input of thecarrier amplifier, and send a second input signal to an input of thepeak amplifier, the second input signal shifted in phase byapproximately ninety degrees from the first input signal.
 9. The systemof claim 8, further comprising an output matching unit configured toreceive an output signal from the peak amplifier and an output signalfrom the carrier amplifier to generate a substantially optimum poweramplifier output power signal at an output stage.
 10. A system for biascontrol of a power amplifier, comprising: a carrier amplifier coupled toan input stage for amplifying an input signal; a peak amplifier coupledto the input stage for amplifying the input signal, the peak amplifierconfigured to receive a voltage control signal for biasing the peakamplifier; a 3dB hybrid coupler configured to receive the input signalfrom the input stage, send a first input signal to an input of thecarrier amplifier, and send a second input signal to an input of thepeak amplifier, the second input signal shifted in phase byapproximately ninety degrees from the first input signal; and an outputmatching unit configured to receive an output signal from the peakamplifier and an output signal from the carrier amplifier to generate asubstantially optimum power amplifier output power signal at an outputstage, the output matching unit including a first quarter wavelengthtransformer coupled to a carrier amplifier output terminal; and a secondquarter wavelength transformer coupled to a peak amplifier outputterminal, an output of the first quarter wavelength transformer, and theoutput stage.
 11. A system for controlling a power amplifier in a mobilehandset, comprising: a carrier amplifier having a carrier input terminaland a carrier output terminal; a peak amplifier having a peak inputterminal, a peak output terminal and a control terminal for receiving avoltage control signal, the peak amplifier configured to vary at leastone characteristic of the power amplifier based upon the voltage controlsignal; a phase shifter, coupled to the carrier input terminal and thepeak input terminal, for generating a peak amplifier input signaldelayed in phase from a carrier amplifier input signal; and an outputmatching unit, coupled to the carrier output terminal and the peakoutput terminal, for transmitting a carrier output power signal and apeak output power signal and forming a power amplifier output powersignal at a power amplifier output stage, the output matching unitincluding a first transformer having an input coupled to the carrieroutput terminal and an output coupled to the peak output terminal; and asecond transformer having an input coupled to the output of the firsttransformer and an output coupled to the power amplifier output stage.12. The system of claim 11, further comprising a baseband modem chipsetfor receiving signals transmitted by a remote base station andgenerating the voltage control signal in a first voltage state if powerlevels of the received signals indicate that the power amplifieroperates within a low power range and generating the voltage controlsignal in a second voltage state if the power levels of the receivedsignals indicate that the power amplifier operates within a high powerrange.
 13. The system of claim 11, wherein the phase shifter is a hybridcoupler for distributing certain input powers to the carrier amplifierand the peak amplifier.
 14. The system of claim 13, wherein the hybridcoupler is a 3dB hybrid coupler implemented with lumped elements. 15.The system of claim 13, wherein the hybrid coupler is implemented by theLow Temperature Co-fired Ceramic (LTCC) method.
 16. The system of claim11, wherein the phase shifter is a phase difference compensator.
 17. Thesystem of claim 16, wherein the phase difference compensator isimplemented with a transmission line.
 18. The system of claim 16,wherein the phase difference compensator is implemented with lumpedelements.
 19. The system of claim 11, wherein the output matching unitis implemented with lumped elements.
 20. The system of claim 11, whereinthe output matching unit is implemented by a Low Temperature Co-firedCeramic (LTCC) method.
 21. The system of claim 11, wherein the at leastone characteristic of the power amplifier is linearity.
 22. The systemof claim 12, wherein the peak amplifier further comprises a voltagecontrol unit configured to receive the voltage control signal andcontrol a bias current of the peak amplifier such that the poweramplifier is operated as a Doherty-type amplifier when the voltagecontrol signal is in the first voltage state and the peak amplifier isoperated as a class AB amplifier when the voltage control signal is inthe second voltage state.
 23. The system of claim 11, wherein the andthe second transformer are both quarter wavelength transformers.
 24. Amethod of operating a power amplifier in a wireless transmitting devicein at least two modes, the power amplifier including a carrier amplifierand a peak amplifier, the method comprising: generating a voltagecontrol signal in a first voltage state if power levels of signalstransmitted by a remote base station and received by the power amplifierindicate that the power amplifier operates within a low power range;generating a voltage control signal in a second voltage state if thepower levels of signals transmitted by the remote base station andreceived by the power amplifier indicate that the power amplifieroperates within a high power range; biasing the peak amplifier via thevoltage control signal, and biasing a carrier amplifier independentlyfrom the voltage control signal.
 25. The method of claim 24, whereinbiasing further comprises the step of biasing the peak amplifier via thevoltage control signal in the first voltage state to operate the poweramplifier as a Doherty-type amplifier.
 26. The method of claim 24,wherein biasing further comprises the step of biasing the peak amplifiervia the voltage control signal in the second voltage state to improve anon-linearity characteristic of the power amplifier.
 27. The method ofclaim 24, wherein biasing further comprises the step of biasing the peakamplifier via the voltage control signal in the second voltage state tooperate the peak amplifier as a class AB amplifier.
 28. A system ofoperating a power amplifier in a wireless transmitting device in atleast two modes, the power amplifier including a carrier amplifier and apeak amplifier, the method comprising: means for generating a voltagecontrol signal in a first voltage state if power levels of signalstransmitted by a remote base station and received by the power amplifierindicate that the power amplifier operates within a low power range;means for generating a voltage control signal in a second voltage stateif the power levels of signals transmitted by the remote base stationand received by the power amplifier indicate that the power amplifieroperates within a high power range; means for biasing the peak amplifiervia the voltage control signal, and means for biasing the carrieramplifier independently from the voltage control signal.
 29. The systemof claim 28, wherein means for biasing further comprises means forbiasing the peak amplifier to operate the power amplifier as aDoherty-type amplifier if the voltage control signal is in the firstvoltage state.
 30. The method of claim 28, wherein means for biasingfurther comprises means for biasing the peak amplifier to improve anon-linearity characteristic of the power amplifier if the voltagecontrol signal is in the second voltage state.
 31. The system of claim1, wherein the carrier amplifier is biased independently from the peakamplifier.